Voltage-gated ion channels play a critical role in shaping of electrical activity of neuronal and muscle cells, and in controlling the secretion of neurotransmitters and hormones through the gating of calcium ion entry. Large families of voltage gated sodium (Na+), potassium (K+) and calcium (Ca2+) ion channels have been defined using electrophysiological, pharmacological and molecular techniques [1, 18]; they are named according to their selective permeability for a particular cation with reference to their voltage dependence, kinetic behaviour or molecular identity. The importance of membrane voltage and ion permeability in the control of cell function ensures that modulation of ion channels will invariably have important consequences for cells and tissues, and such modulation can often be turned to therapeutic advantage. Major indication for ion channel modulators already include cardiac arrhythmia, hypertension, anxiety, epilepsy, pain, chemotherapy-induced nausea and diabetes as well as a range of drugs in development for important new indications such as neuroprotection and psychiatry.
A variety of bodily functions such as heart beat, sleep-wake cycles, secretion of hormones and control of behavioural state depend on the action of pacemakers, specialised cells that are able to generate rhythmic, spontaneously firing action potentials. The archetypal organ displaying autonomic rhythmicity is the heart.
Pacemaking in the heart is accomplished by the rhythmic discharge of the sino atrial node [8, 11, 12]. The firing rate of the sino atrial node is determined by the diastolic depolarisation phase of the action potential. During this phase the membrane potential is slowly depolarised to the threshold triggering the next action potential. The ionic conductance underlying the cardiac pacemaker depolarisation was identified in the late seventies and early eighties [10] and called If (f for ‘funny’) or Ih (h for hyperpolarisation activated). A similar current was subsequently discovered in neurones, first in photoreceptors, [2, 3, 6] and then in various central neurons eg hippocampal pyramidal cells [16] where is was called Iq (q for ‘queer’). This current was subsequently found in a wide variety of central and peripheral neurons [25].
Ih channels have several distinctive features. Unlike most voltage-gated channels, they open in response to negative-going voltage steps to potentials within the range of the normal resting potential. They conduct both K+ and Na+ ions with a three fold greater permeability to K+, yielding a reversal potential of −30 to −40 mV under physiological conditions. As a result, the opening of these channels near the resting potential (˜−60 mV) generates an inward, depolarising current that is largely carried by Na+ [25]. Another unusual property of these channels is their regulation by cyclic nucleotides [11], which speed up the rate of channel activation by binding to an intracellular site on the channel. In the heart, this in an important mechanism responsible for the acceleration of heart rate in response to sympathetic stimulation. Activation of β-adrenergic receptors leads to an activation of adenylyl cyclase with the resulting increase in intracellular cAMP directly activating the Ih channel. cAMP binding leads to a shift in the activation curve towards more positive voltages. This shift results in an increased inward current at a fixed membrane potential and therefore an acceleration of the diastolic depolarisation [9]. Muscarinic stimulation slows the heart rate, in part due to a decrease in cAMP level and a resulting reduction in the Ih current [13, 33].
In neurons, Ih channels have diverse functions. They were initially shown to be inward rectifiers; they are active near the resting potential and pass inward current more readily than outward current, thereby helping to control of resting potential and input resistance [25]. In photoreceptors, Ih channels help to damp the hyperpolarising effect of light; they are activated by hyperpolarisation, causing the voltage response to light to fade during the first 100-200 ms, thus producing sensory adaptation. In many CNS neurons, activation of Ih channels following post inhibitory post synaptic potentials contributes to a rebound afterdepolarisation (ADP), which can trigger an action potential. Ih is also generates or contributes to ‘pacemaker’ potentials that controls the rate of rhythmic oscillations, similar to its role in the heart. Ih has been found to regulate the rhythmic activity of thalamic relay neurons [23] and inferior olivary neurons [5] through interaction with a T-type calcium current. The oscillating single neurons are part of neuronal networks and are involved in the generation and modulation of rhythmic activity of these networks. A well studied example are spindle waves observed in the EEG during slow wave sleep, which are generated through interactions between thalamic reticular and relay neurons [4]. Regulation of Ih in these cells is important in the sleep-wake cycle. Although less well investigated, results suggest a similar role for Ih in the generation of oscillations in hippocampal neurons [21, 32] and respiratory neurons of the preBotzinger complex of the ventrolateral medulla [26]. Ih channels are also expressed in dendrites where they influence the cable properties of the dendrite and shape the time course of the EPSP as it is propagated to the soma [22]. A recent study has extended the role of Ih neurons by showing that these channels can alter neurotransmitter release from presynaptic terminals as Crayfish neuromuscular synapses. Presynaptic cAMP generation by via serotonin receptor activation directly modulates Ih in axons that produces an increase in synaptic strength which cannot be explained solely by depolarisation of the presynaptic membrane [7].
The genes encoding Ih channels were recently cloned from both mammals [19, 28, 29] and sea urchins [15]. These genes, called HCN1-4 in mammals, are members of the voltage gated K+ channel family. The encoded proteins contain six transmembrane segments, including a positively charged S4 voltage sensor and a pore-forming P region that includes the K+ channel signature sequence GYG. In addition, the C-terminus contains a 120-amino acid sequence that is homologous to the cAMP and cGMP binding domains of other proteins, and is therefore the likely site for cAMP regulation of channel opening [9,20]. All four mammalian genes are expressed in brain, with differing expression patterns [24]. While the first reported cloning of Ih was from sea urchin spermatozoa [15], the functional significance is poorly understood at present. The channel is expressed in sperm flagellum and it was postulated that it may be involved in the control of flagella beating. One of the cloned isoforms, HCN4, has been detected in testes suggesting that Ih may also have a function in mammalian testicular and sperm function [30].
These ion channels are the target for blocking molecules for therapeutic use in dysfunction in the CNS, cardiovascular dysfunction of the heart, and reproductive dysfunction and/or contraception related to Ih function in testes and spermatozoa. For instance, compounds have already been developed with the therapeutically interesting property of inducing bradycardia with minimal inotropic side effects [14, 17]. Unfortunately, there are no compounds yet described that distinguish cardiac from neuronal isoforms, and volunteers experienced optical hallucinations probably due to reduced functionality of Ih in photoreceptors. It is clear the ability to develop agents that are selective for the CNS vs heart or vice versa requires the availability of the cloned subunits to screen for compounds selective for subunits expressed in either neuronal or cardiac tissue.